INFO-0545 RADIOISOTOPE SAFETY MONITORING FOR RADIOACTIVE CONTAMINATION

Transcription

1 INFO-0545 RADIOISOTOPE SAFETY MONITORING FOR RADIOACTIVE CONTAMINATION 1. INTRODUCTION This document provides general guidance for monitoring and controlling radioactive contamination, and relating the monitoring results to the AECB radioisotope licence criteria. This document pertains primarily to laboratories, but also applies to the control of radioactive contamination in other locations The document also provides guidance on contamination monitoring instrument selection. Each AECB radioisotope licence authorising the use of open source radioactive material contains a condition which states the regulatory criteria pertaining to radioactive contamination. This licence condition is stated in Annex I. The specified contamination criteria must be applied to all areas where radioisotopes are used. Notwithstanding these limits, licensees should maintain levels or radioactive contamination as low as reasonably achievable (ALARA). 2. ELEMENTS OF A CONTAMINATION MONITORING PROGRAM 2.1 Method of Measurement Radioactive contamination may be measured directly or indirectly. Direct measurement means the use of portable radiation detection instruments to detect both fixed and removable contamination. Direct measurement may be used when background radiation levels are negligible compared to licence criteria. Indirect measurement only detects removable contamination by means of wipe tests. 2.2 Instrument Selection The ability of various radiation detection instruments to detect radioisotopes of interest will vary with the instrument and manufacturer. Guidance on the selection of instruments is given in a table in Annex 2. For specific information on a particular make or model, contact the manufacturer. 2.3 Locations of Measurement The locations that are to be monitored should be numbered on a plan of the radioisotope work area. These locations should include working surfaces, such as benches, countertops, fume hoods, etc.. storage areas, and non-working surfaces such as floors, instruments and equipment, door handles, light switches, sink taps and telephone receivers. Several random locations should also be monitored. Too rigid a set of locations may overlook problem areas. Commission de contrôle de l énergie atomique Atomic Energy Control Board

2 2.4 Instrument Checks and Calibration Non-portable instruments used for counting wipes, such as liquid scintillation counters, well-crystal type gamma counters, gas-flow proportional counters, semiconductor gamma spectrometers and gamma cameras, should be routinely serviced according to the manufacturer's instructions. Keep a record of the service information and dates. Before monitoring for contamination, portable instruments should be given operational checks as specified by the manufacturer (i.e. battery check, high-voltage check, response check, etc.) and the background radiation level should be measured. Record the operational checks and background measurement. Similarly, non-portable instruments used to count wipes should count and record a blank and standard with each set of wipes. Instruments that are not operating within the parameters of the operational checks or which show anomalous background, blank or standard measurements, should not be used until their proper operation can be verified. 2.5 Frequency of Monitoring Contamination monitoring frequencies must conform to the requirements indicated on the appropriate AECB Laboratory Rules Poster, or in a radioisotope licence condition. When radioactive material is not used for a prolonged period of time, contamination monitoring is not required, but such a period should be identified in the records. 2.6 Decontamination Any area that is found to have non-fixed contamination exceeding the regulatory criteria must be cleaned and remonitored. If the area cannot be cleaned to meet the criteria, the contaminated surface must be sealed, removed or shielded until the criteria are met. NOTE: For short-lived radionuclides, the room or area may be posted and secured until the radioisotope decays. 2.7 Monitoring Records Contamination monitoring records must be kept for three years and must be available for inspection by AECB staff. These records should include: a) date of measurement b) make and model of the instrument c) monitoring locations d) contamination monitoring results in Bq/cm 2 (before and after decontamination)

3 e) for portable instruments, the results of operational checks and background measurements f) for non-portable instruments, blank and standard measurement results Instrument calibration data should be recorded and updated as necessary. 3. DIRECT MEASUREMENT OF CONTAMINATION USING A PROTABLE METER Depending upon the detector and the radioisotopes, direct measurement is often convenient for monitoring large areas. Direct measurement instrument readings include both fixed and non-fixed contamination. Thus a reading which satisfies the licence criteria gives a conservative estimate of non-fixed contamination. a) Monitor the locations marked on the plan or the working area by slowly passing the detector over each area. b) Keep the detector face towards the surface being monitored and keep the distance between the detector and surface as small as possible without touching (and possibly contaminating) the detector. c) If contamination is detected, stop and obtain a measurement. Clean the area until the instrument measurement is below the licence criteria. A reading in excess of licence criteria after repeated cleaning is an indication or fixed contamination or a high radiation background. d) Identify and mark the contaminated area on the plan e) Record the highest measurement for each area and the final measurement after decontamination 4. INDIRECT MEASUREMENT OF CONTAMINATION WITH WIPES a) Wipe each of the locations shown on the plan of the working area with a filter paper, wipe or cotton swab lightly moistened with alcohol or water. Use one numbered wipe per location. (Note 1 ) b) Wipe an area of 100 cm 2. Using uniform and constant pressure, ensure the entire area is wiped. c) If necessary, carefully dry the wipe to prevent loss of activity. (Note 2) d) Count the wipes in a low-background area and record all results. e) If the wipes are to be counted on a contamination meter, the wipe should be smaller than or equal to the sensitive area of the detector. f) Clean any contaminated areas and remonitor. Record results before and after decontamination. NOTES: 1) One "screening" wipe can be used to monitor several locations. If contamination is found, the contaminated area must be identified and decontaminated. 2) Since the contamination may be absorbed into the wipe material, the use of a wetting agent may lead to a significant underestimate of alpha and low-energy beta contamination with some counting methods.

4 5. RELATING MEASUREMENT READINGS REGULATORY CRITERIA The readings from contamination meters and non-portable instruments can be related to regulatory criteria if the efficiency of the instrument for a spedfic radioisotope is known. Instrument efficiencies for specific radioisotopes can be obtained from the manufacturer or determined using an appropriate standard of known activity (For a description of instrument efficiency. see Annex III). For mixtures of radioisotopes, do all calculations using the radioisotope for which the instrument has the lowest detection efficiency. Using the following equation, calculate the measurement results in Bq/cm 2 Removable Activity = N - NB Ex60xAx(F) WHERE: N = is the total count rate in counts per minute (CPM) measured directly or on the wipe NB = is the normal background count rate (in CPM) from the survey instrument or on the blank E = is the instrument efficiency factor (expressed as a decimal, i.e. for 26% efficiency, E=0.26) for the radioisotope being measured (consult the manufacturer or determine using a radioactive source with a known amount of activity in a counting geometry similar to that used when surveying for contamination) (see Annex 3) 60 = sec/min A = F = area wiped (not to exceed 100 cm 2 ) or area of the detector in cm 2 (for direct measurement) is the collection factor for the wipe (used ONLY when calculating indirect wipe monitoring results) If F is not determined experimentally, a value of F = 0.1 (i.e. 10%) shall be used. 6. INSTRUMENT SENSITIVITY Portable contamination monitoring instruments must be capable of making reproducible measurements at the licence criteria limits. A minimum detection limit is approximately twice the background measurement.

5 7. FURTHER INFORMATION For further information about the implementation of this guide, contact the AECB at the following address: Atomic Energy Control Board Radioisotope and Transportation Division P.O. Box 1046 Ottawa, Ontario K1P 5S9 Telephone: (613) Facsimile: (613)

6 ANNEX 1 AECB Radioisotope Licence criteria for radioactive contamination: The licensee shall ensure that: a) on all normally accessible working surfaces in any location where a radioactive prescribed substance is used or stored, non-fixed contamination does not exceed 0.5 becquerel per square centimetre of alpha activity or 5 becquerels per Square centimetre of beta or gamma activity, averaged over an area not exceeding 100 square centimetres; b) on all other surfaces, and prior to decommissioning any location where a radioactive substance has been used or stored, non-fixed contamination does not exceed 0.05 becquerel per Square centimetre of alpha activity or 0.5 becquerel per square centimetre or beta or gamma activity, averaged over an area not exceeding 100 square centimetres; c) the dose rate due to fixed contamination does not exceed 0.5 microsievert per hour at 0.5 metre from any surface; and, d) records of all contamination measurements shall be maintained for at least three years.

7 ANNEX II SELECTION OF CONTAMlNATION MONITORING INSTRUMENTS The following is a general list of the various types of contamination monitoring instruments for some commonly used radioisotopes. Please consult the manufacturer for the exact specification of the instrument your are interested in. HAND-HELD CONTAMINATION MONITORING INSTRUMENT NON-PORTABLE MONITORING INSTRUMENTS (WIPE COUNTERS) Type of Instrument Code Type of Instrument Code Thin-window G-M detector 1 Gas-Flow proportional counter A Ion chamber with beta window 2 Liquid scintillation counter B Gas-filled proportional detector 3 Well-crystal sodium iodide counter C Thin-Iayer sodium iodide scintillation detector 4 Semiconductor gamma spectrometers D Thick-crystal sodium iodine scintillation detector 5 Organic crystal/plastic scintillation detector 6 Zinc sulphide scintillation detector 7 COMMON RADIOISOTOPES AND SUGGESTED MONITORING SELECTION Radioisotope Half-Life Principal Emission Energy (kev) Hand-Held Instruments Non-Portable Instruments H-3 12 years beta B C years beta A B P days beta A B S days beta A B Ca days beta A B Cr days photon 320(10%) 3 6 B C D Mn days photon 835 (100%) B C D Fe-55 3 years Electron Capture 5.9 (25%) 6 B C D Co days photon 122 (86%) B C D Co-60 5 years beta A B C D Ni years beta A B Ga-67 3 days photon 93 (36%) A B C D Rb days beta A B C D Sr years beta A B Tc-99m 6 hours photon 141 (89%) A B C D In days photon 245 (94%) A B C D I days photon 35 (6.5%) B C D I days beta A B C D Ba years photon 356 (60%) A B C D Cs years beta A B C D Ir days beta A B C D TI days photon 167(10%) A B C D Pb years beta A B C D Ra years alpha A B C D

8 NOTE: a) The principal energy quoted for beta emitters is the average beta energy. b) The (%) beside the gamma emitters is the percent abundance for that energy. c) Certain radioisotopes, such as Po-210 and Ra-226, are part of a decay chain and can be detected by measuring for their decay products. Ra-226 plus its daughters is an alpha, beta and gamma emitter and can be efficiently detected using a thin-window geiger detector, a well-crystal sodium iodide counter or a liquid scintillation detector. d) Some hand-held instruments may also be used to screen wipes if used in a low-background area.

9 ANNEX III DETECTOR EFFICIENCY The detector efficiency depends upon: the type of detector (GM, Nal Scintillation, Plastic Scintillation, Proportional) the detector size and shape (larger areas and volumes are more sensitive) the distance from the detector to the radioactive material the radioisotope and type of radiation measured (alpha, beta and gamma radiations and their energies) the backscatter of radiation toward the detector (the denser the surface, the more scattering) the absorbtion of radiation before it reaches the detector (by air and by the detector covering) The factors affecting the efficiency are shown in the diagram below The detector efficiency can be found by: 1. Counting a standard source of known activity with your detector. efficiency = (detector count rate -background count rate) known activity of standard source 2. Asking the manufacturer about the efficiency of the detector for your specific radioisotope(s). (1) Some radiation goes directly from the radioactive material P, into the detector. (2) Some radiation will backscatter off the surface, into the detector. (3) Some radiation is absorbed by the detector covering. (4) Most radiation doesn't even get detected. (5) If the detector was closer, this radiation would be detected.